The present invention is directed to an inspection system for ultrasonically inspecting a material such as metal, and, more particularly, the present invention is directed to a boresonic inspection system which performs shear mode inspection of near bore material in turbine and generator rotors by passing ultrasonic search units through an axial rotor bore and locates flaws.
In a conventional contact ultrasonic system, the emitted wave travelling through the rotor material is divergent; that is, the wave front grows in size as it moves away from the source. The intensity of this wave therefore decreases with increasing travel distance (since the area covered by the wave is increasing) and therefore, the intensity of a wave returning from a given reflector decreases with increasing distance of the reflector from the search unit Also, since most reflectors are small relative to the area (beam size) covered by the wave, the size of the reflector affects the intensity of the reflected wave. These principles have been long known and understood in ultrasonic testing in general and are used to provide an estimate of the size of an unknown reflector. The intensity of a reflected wave is normally converted, through the piezoelectric property of the transducer element, to a voltage which is then linearly presented as a signal amplitude on a cathode ray tube type presentation. Distance/Amplitude and Area/Amplitude relationships are determined using known reflectors in reference standards under conditions which reproduce or, at least, simulate the prevailing test conditions (bore curvature, attenuation, etc.). The total inspection system, including the search unit, transmit and receive electronics, amplifiers, displays, cables, etc., are calibrated using the known, artificial reflectors in a reference standard. Reflectors are considered to be reportable when their amplitudes exceed a specific amplitude limit which normally includes the Distance/Amplitude correction. Its size is estimated using the established Area/Amplitude relationship.
Some of the disadvantages associated with boresonic test systems using contact transducers are as follows:
1. Contact inspection is limited in its ability to accurately size real reflectors. Ideal reflectors, commonly flat bottom holes (with the beam normal to the flat) and side drilled holes (with the beam normal to the hole axis), are used to develop the Distance/Amplitude and Area/Amplitude relationships used for detecting and sizing reflectors with a divergent beam transducer. Since any given reflector geometry has its own reflectivity, these relationships are only valid for the type of reflector from which they were derived. Therefore, the use of a given set of relationships developed on, for example, flat bottom holes will not be accurate for other Specific geometries such as spheres, off-axis discs, elliptical notches, etc. The problem is compounded further when considering irregular, randomly oriented reflectors characteristic of real reflectors in real materials. It is for this reason that size estimates for such reflectors are normally given in terms of equivalency to the ideal reflectors used to develop the calibration relationships (such as Equivalent Flat Bottom Hole Area) and do not necessarily reflect the actual size.
2. Resolution, as used in ultrasonic testing, is defined as the ability to discriminate between two reflectors lying in close proximity to one another. Because the ultrasonic beam in a contact system is divergent, resolution is very poor. This means, for example, that a number of relatively small reflectors could be reported as one larger reflector, an error which could affect the analysis and final disposition of the rotor.
In the prior art, manual, pneumatic and motor driven inspection systems the control systems that move the scan head and provide position indications have been cumbersome and inaccurate due to resolver locations that require knowledge of mechanical slack in the system and positioning apparatus that does not allow for high resolution positioning. As a result, the location and size of discontinuities and flaws have been inaccurately located. Inaccurate flaw location, requires that remachining to remove flaws cover a larger area than is necessary, weakening the rotor at its highest stress area, near the bore. Inaccurate flaw location also hinders comparison of previous inspections with current inspections because it is difficult to determine whether a given flaw is a new flaw or an old flaw that has been inaccurately located due to alignment inaccuracies.
See U.S. Pat. No. 4,757,716 for additional discussion relating to the background of this invention.